I thought I'd start a thread on MotorAll about this, even though the thread on NAM is not so bad. Perhaps this will become a Library thread if we can pull together some common data.

The photo below shows my final system, which includes a large volume perforated and packed muffler and a packed terminated resonator, using the Helmholtz principle, to attenuate the annoying resonance/drone around 3200 rpm. This system is 2.5" starting with a custom CMC header (4-2-1, ceramic coated inside and out), 200cell race cat, Hushpower II resonator and the muffler you see in the photo.

In order to install a straight exhaust in a R53 you must relocate the battery and remove the battery box. There's another thread on that: Battery Delete

That's a Helmholtz resonator, Dr Mike. An audio concept, used in taming resonance frequencies in speakers, and recording studios/listening rooms, for example. It's a tuned closed-end column of air, designed to absorb acoustic energy of a particular frequency. In this case, the exhaust system "drones" at a particular rpm, and the Helmholtz resonator is tuned to the associated frequency and absorbs the energy at that frequency.
So the exhaust sounds normal (loud, growly, etc) at all rpm/frequencies and rather than being louder at the drone, it's about normal thanks to the Helmholtz.
It actually works. I've seen then on OEM Honda S2000 exhausts, on aftermarket Z350 exhausts and there's a big thread about them on some Mustang forums. The cross-pipe on Corvette exhausts and other v-8 systems acts in a similar way to attenuate certain frequencies.

As for the thread referred to on the other site, there's another one there also. There's a lot of good information in those threads, for sure. I just thought I'd put one here, to see if we get some new ideas.
When I get some time and soon, I'll add the battery box delete pre-amble and tell the whole story in one place. Then maybe we can make it a Library thread. I get the idea that Library threads are more meta-analysis and composites of all the ideas on a particular topic.